The invention proposes a method for compensating for propagation inhomogeneities affecting a time-domain reflectogram which is measured for a given cable by injecting a test signal, called the reflectometry signal, into this cable and then acquiring this signal reflected from any impedance singularities or discontinuities present in the cable.
The invention is applicable to any type of electric cable, particularly power transmission cables or communication cables, in fixed or mobile installations. The cables in question may be coaxial or two-wire, in parallel alignment or twisted pairs, stranded, or other types.
Time-domain reflectometry is a method commonly used in the diagnosis of cables and wired networks. This method consists in injecting a signal into a point-to-point cable or a cable network and then measuring the returned echoes created by reflections of the injected signal from singularities in the cable. The delay and amplitude of these echoes can be used to obtain information on the structure or on the electrical faults present in the cable or cable network, thus enabling a breakdown or, more generally, an electrical fault to be located, characterized, and predicted if necessary.
When a reflectometry system is used, it is frequently found that the measurement of the reflected signal, called a reflectogram, is perturbed by various noise sources.
The common, externally originating, measurement noise, for example that caused by parasitic elements due to the environment of the cable or network, should be distinguished from internally originating noise such as spontaneous fluctuations caused by the discrete nature of phenomena at microscopic level, background noise, thermal noise, and pulsed perturbations. This noise is random in nature and varies stochastically over time.
Another noise, called propagation noise, may also be superimposed on the useful measurement of the reflectogram. This is due to inhomogeneities in the cable, distributed randomly over its length, which cause minor local reflections of the signal and create undesirable small echoes in the reflectogram.
More generally, the measured reflectogram may show undesirable echoes due to these inhomogeneities or to other phenomena such as the reflection of the signal from a junction or branch, a connector, a cable end load or any other device linked to the cable, for example an external device such as a busbar, a cable duct or a cable tray. These echoes may be confused with peaks characteristic of faults which are to be identified by the reflectometry method; they may also mask these echoes because their amplitude is greater than that of the fault peaks being searched for.
The analysis of the reflectogram and the identification of the faults being searched for may therefore be falsified by these undesirable peaks due to the propagation inhomogeneities.
The invention consists in designing an optimal test signal which is specifically adapted to the inhomogeneities present in a cable and which can be used to “clean” the reflectogram. After the injection of this adapted test signal according to the invention, the resulting reflectogram will no longer contain any peak except that which corresponds to the possible impedance mismatch at injection point. The appearance of a fault after the injection of this adapted signal will therefore be very clearly visible, and its location will be greatly facilitated.
It is also possible to generate an adapted signal which results in the measurement of a reflectogram which is not free of all peaks, but has the peaks whose appearance is desirable (if this is of any interest). These peaks may be the peaks that it is desirable to retain, or peaks that can be used to set markers for assistance in subsequent fault location.